Sensitivity of a Coarse‐Resolution Global Ocean Model to a Spatially Variable Neutral Diffusivity. (25th March 2022)

• Record Type: Journal Article
• Authors: Holmes, R. M.; Groeskamp, S.; Stewart, K. D.; McDougall, T. J.
• Is Part Of: Journal of advances in modeling earth systems. Volume 14:Number 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-25
• Subjects: ocean modeling -- neutral diffusion -- mesoscale mixing -- parameterization; Geological modeling -- Periodicals; Climatology -- Periodicals; Geochemical modeling -- Periodicals; 551.5011
• DOI: 10.1029/2021MS002914
• Languages: English
• ISSNs: 1942-2466

Abstract

Abstract: Motivated by recent advances in mapping mesoscale eddy tracer mixing in the ocean we evaluate the sensitivity of a coarse‐resolution global ocean model to a spatially variable neutral diffusion coefficient κ n ( x, y, z ). We gradually introduce physically motivated models for the horizontal (mixing length theory) and vertical (surface mode theory) structure of κ n along with suppression of mixing by mean flows. Each structural feature influences the ocean's hydrography and circulation to varying extents, with the suppression of mixing by mean flows being the most important factor and the vertical structure being relatively unimportant. When utilizing the full theory (experiment "FULL") the interhemispheric overturning cell is strengthened by 2 Sv at 26°N (a ∼20% increase), bringing it into better agreement with observations. Zonal mean tracer biases are also reduced in FULL. Neutral diffusion impacts circulation through surface temperature‐induced changes in surface buoyancy fluxes and nonlinear equation of state effects. Surface buoyancy forcing anomalies are largest in the Southern Ocean where a decreased neutral diffusivity in FULL leads to surface cooling and enhanced dense‐to‐light surface water mass transformation, reinforced by reductions in cabbeling and thermobaricity. The increased water mass transformation leads to enhanced midlatitude stratification and interhemispheric overturning. The spatial structure for κ n in FULL is important as it enhances the interhemispheric cell without degrading the Antarctic bottom water cell, unlike a spatially uniform reduction in κ n . These results highlight the sensitivity of modeled circulation to κ n and motivate the use of physics‐based models for its structure. Plain Language Summary: The diffusion of tracers such as temperature and salinity along surfaces of constant density by the action of mesoscale eddy mixing, known as neutral diffusion, is an important transport process in the ocean which impacts heat, carbon, and nutrient budgets as well as climate variability. However, most global ocean circulation models used for climate studies have a horizontal grid resolution that is too coarse to resolve mesoscale eddies. Thus, the effects of eddy‐driven neutral diffusion must be parameterized through the inclusion of a neutral diffusivity parameter κ n . While the strength of neutral diffusion is known to vary spatially within the ocean, most models still make simple choices for κ n ; a constant, or scaled according to the grid resolution. In this study, we examine the sensitivity of a coarse‐resolution global ocean model to the spatial structure of κ n using a recently developed and physically motivated three‐dimensional mapping of mesoscale mixing. Our results show that the modeled meridional overturning circulation and tracer structure are sensitive to both the magnitude and the spatial structure of κ n, suggesting that more attention should be paid to this parameter in future model development. Key Points: A new spatially variable parameterization for mesoscale neutral diffusion is tested in a 1‐degree ocean model Both hydrography and circulation are sensitive to the magnitude and spatial structure of the neutral diffusivity A 2 Sv enhancement in interhemispheric overturning stems from suppression of neutral diffusion by mean flows